Preheat oven to 350 degrees F (175 degrees C). Grease a 2-quart baking dish.
Cook macaroni according to package directions; drain.
In a large saucepan, melt butter over medium heat. Stir in flour, salt and mustard. Gradually add milk; cook and stir until thickened. Remove from heat; stir in 1-1/2 cups cheese until melted.
Combine macaroni and cheese sauce; spoon into prepared baking dish. Sprinkle with remaining cheese.
Bake 25 minutes in the preheated oven, or until bubbly and golden brown.